The present invention relates to a method for the proper alignment of sheets before the transfer of the sheets to a sheet-processing machine, particularly a printing press. A device for carrying out this method is called a stream feeder.
German Patent Application No. 44 36 034 discloses a sheet-alignment method in which the sheet, irrespective of the initial sheet position, is supposed to arrive at front or side guides with as low a speed as possible. To that end, a normalized motional function is generated in advance for the carrier, and is stored in the memory of an electronic control device. During operation, the deviation of a sheet from a setpoint position is detected by a sensor, and a signal corresponding to the deviation is generated. Using the signal and the normalized motional function, an individual motional function is calculated in the control device for each sheet. In dependence upon the individual motional function, the control device generates driving commands for the motor and moves the carrier accordingly.
In addition, there are methods for sheet alignment without the aid of aligning stops, as described in the German Patent No. 33 01 722, for example. After the sheet has passed a measuring zone for detecting the position, it is moved a predefined, constant-remaining distance into the register-true position.
Highly precise position sensors are needed for the methods described above, as well as, in the first case, costly on-line calculations, and the mechanism for the transport of sheets into the position, aligned true to register, must likewise function very precisely and reliably.
In contrast, simple, mechanical feeders function, for example, in the following manner. After striking against front guides, the sheet is pressed by a xe2x80x9ctiming rollerxe2x80x9d onto an oscillating transport roller or draw rail that is synchronized with the machine drive, i.e. is permanently coupled to the main drive of the machine. Due to the frictional connection attained, the sheet is moved against a side guide, in order to achieve a register-true alignment of the sheet edge abutting against the side guide.
However, this method has several disadvantages: Since the draw path is constant, but the initial position of the sheet will inevitably be different because of inexactness in the stacking, in principle, the sheet must be drawn somewhat further than corresponds to the position of the aligning stop. A deformation of the sheet is counteracted by defined slippage, adjustable by hand, between the transport roller and draw rail, respectively, and the seated timing roller. This process is faulty and, given critical printing materials and/or higher printing speeds, cannot be easily controlled. Thus, the sheet edge can become damaged. Furthermore, in response to an abrupt stop at the side guide, the sheet is subject to forces which can cause it to twist uncontrolled in its plane and/or to partially bulge, so that an originally exact leading-edge alignment deteriorates. To restore the exact alignment of the leading edge of the sheet in some way would be very difficult from a mechanical standpoint and at the same time prone to error.
The present invention provides a sheet-transport method in which a signal is generated when a sheet edge to be fed passes a predetermined location between a carrier and the aligning stop, and that in response to the signal, the carrier performs a preset movement, identical from sheet to sheet, by which the carried-along sheet is fed to the aligning stop with a speed greater than zero, but substantially less than the maximum sheet speed during the feeding movement.
It is not necessary to transport the sheet with the assistance of the carrier as exactly as is only possible in its final position in the feeder, since an aligning stop is used. If the target position of the carrier lies a little beyond the aligning stop, the sheet has already lost considerable speed when it reaches the aligning stop. Given such a relatively low speed, there is no longer any danger that the sheet will twist or become deformed when it reaches the aligning stop. To achieve this, the sheet does not have to be moved an exactly defined distance, which is only executable with high expenditure; rather, it is sufficient if the carrier carries out an exactly defined movement, which can be achieved in a very much simpler manner, e.g. using a simple electromechanical drive. Only the starting instant of the carrier movement is determined by a normally electronic sensor. However, the demands on the measuring accuracy of the sensor would be considerably less if, subsequently, a positioning which is as precise as possible were to be carried out without, or at, side guides.
The movement course of the carrier is so selected that the speed is reduced shortly before reaching the aligning stop. In this manner, the sheet is fed gently to the aligning stop, without the working speed of the feeder being noticeably reduced. The distance of the preset carrier movement must be so large that each sheet reaches the aligning stop, regardless of its position on the feeder pile. Thus, this distance must be selected as a function of the exactness of the sheet alignment within the feeder pile. To be on the safe side, it can be adjusted to be somewhat greater than would correspond to the anticipated, maximum sheet misalignment within the feeder pile, so that even a sheet which is considerably displaced away from the aligning stop will reach the stop with certainty.
It may be that a sheet displaced toward the aligning stop is drawn further by the carrier than would correspond to the stop position. However, because of the reduced speed at this instant, it does not result either in damage to the striking sheet edge, or to twisting or distortion of the sheet, if the slippage between the carrier and the sheet is suitably adjusted. Therefore, the sheet alignment does not deteriorate, even in response to great displacements within the feeder pile. In turn, this means that the pile alignment does not have to be so precise, i.e. great alignment tolerances are possible.
The present invention is particularly suited for a side-edge alignment at an aligning stop made up of one or a plurality of side guides, since the exactness of a previous leading-edge alignment is maintained according to the present invention. Alternatively or in addition, however, the present invention is also suitable for a precise leading-edge alignment that is gentle to the sheets at an aligning stop composed of one or a plurality of front guides.
Preferably, the carrier comprises a conveyor element having a carrier surface that is movable in the sheet plane, for instance a transport roller, which is driven by a motor, or a linearly movable element that is driven by a linear-motion motor. As usual, the sheet is carried along due to friction between the carrier surface and the sheet. To produce the necessary friction, a freely rotating pressing roller, also called a timing roller, moves within the machine cycle toward the conveyor element and away from it. Alternatively, or in addition, suction forces can also be used.
The signal for starting the defined carrier movement is generated preferably by a light barrier arranged in the sheet transport path. It should be assured that each sheet pulled off the feeder pile reaches the light barrier and interrupts it.
During the sheet transport by the carrier, the sheet side that is remote from the carrier can be stretched by a second carrier, as is known e.g. from the German Patent No. 33 11 197, in order to slightly stretch the lead edge of the sheet before the transfer to a gripper system of the sheet-processing machine or printing machine.
According to a further development of the present invention, the force which the carrier exerts on a sheet being carried along decreases before the sheet is fed to the aligning stop. Therefore, this force during the phase of the positive acceleration of the sheet by the carrier can be adjusted to be so high that the sheet is reliably prevented from sliding on the carrier. During the phase shortly before the arrival at the aligning stop, this force is reduced, which can be controlled in a similar manner as the speed reduction, e.g. by a signal from a light barrier arranged in the sheet-transport path. This light barrier can possibly be the same as that for starting the defined carrier movement. To reduce the force in the case of a carrier working with suction force, e.g. of a suction roller or of a suction rail, the negative pressure produced can simply be decreased for a short duration. In the case of a flexibly mounted carrier such as a draw roller or draw rail, the pressing force of the spring or its base point can be adjusted by a suitable actuator as soon as the signal is output for reducing the force.
This further development of the present invention is advantageous, since an analysis of the alignment process reveals that the force which acts on the sheet edge when the sheet strikes is determined not only by the arrival speed of the sheet at the aligning stop, but also by the force with which the sheet is retained by the carrier for the transport to the aligning stop. In the case of frictionally-engaged connections, this force is composed of a normal force and a friction coefficient. By reducing this force shortly before reaching the aligning stop, the sheet can be particularly reliably prevented from twisting or becoming deformed when it reaches the aligning stop.